1. Field of the Invention
The present invention relates to a substrate holding apparatus for holding a substrate to be polished and pressing the substrate against a polishing surface, and more particularly to a substrate holding apparatus for holding a substrate such as a semiconductor wafer in a polishing apparatus for polishing the substrate.
2. Description of the Related Art
In a manufacturing process of a semiconductor device, a thin film is formed on a semiconductor device, and then micro-machining processes, such as patterning or forming holes, are performed. Thereafter, the above processes are repeated to form thin films on the semiconductor device. Recently, semiconductor devices have become more integrated, and structure of semiconductor elements has become more complicated. In addition, the number of layers in multilayer interconnections used for a logical system has been increased. Therefore, irregularities on a surface of the semiconductor device are increased, so that a step height on the surface of the semiconductor device becomes larger.
When irregularities of a surface of a semiconductor device are increased, the following problems arise. Thickness of a film formed in a portion having a step is relatively small. An open circuit is caused by disconnection of interconnections, or a short circuit is caused by insufficient insulation between layers. As a result, good products cannot be obtained, and a yield is reduced. Further, even if a semiconductor device initially works normally, reliability of the semiconductor device is lowered after a long-term use. At a time of exposure during a lithography process, if an irradiation surface has irregularities, then a lens unit in an exposure system is locally unfocused. Therefore, if the irregularities of the surface of the semiconductor device are increased, then it is difficult to form a fine pattern on the semiconductor device.
Thus, during a manufacturing process of a semiconductor device, it is increasingly important to planarize a surface of the semiconductor device. The most important one of planarizing technologies is chemical mechanical polishing (CMP). In chemical mechanical polishing using a polishing apparatus, while a polishing liquid containing abrasive particles such as silica (SiO2) therein is supplied onto a polishing surface such as a polishing pad, a substrate such as a semiconductor wafer is brought into sliding contact with the polishing surface, so that the substrate is polished.
This type of polishing apparatus comprises a polishing table having a polishing surface constituted by a polishing pad, and a substrate holding apparatus, such as a top ring or a carrier head, for holding a semiconductor wafer. When a semiconductor wafer is polished with this type of polishing apparatus, the semiconductor wafer is held by the substrate holding apparatus and pressed against the polishing pad under a predetermined pressure. At this time, the polishing table and the substrate holding apparatus are moved relatively to each other to bring the semiconductor wafer into sliding contact with the polishing surface, so that the surface of the semiconductor wafer is polished to a flat mirror finish.
If a pressing force produced between the semiconductor wafer and the polishing surface of the polishing pad is not uniform over an entire surface of the semiconductor wafer, then the semiconductor wafer is insufficiently or excessively polished depending on the pressing force applied to the semiconductor wafer. Therefore, it has been attempted that a holding surface of the substrate holding apparatus is formed by an elastic membrane of an elastic material such as rubber, and a fluid pressure such as air pressure is applied to a backside surface of the elastic membrane to make uniform the pressing force applied to the semiconductor wafer over the entire surface of the semiconductor wafer.
The polishing pad is so elastic that the pressing force applied to a peripheral portion of the semiconductor wafer becomes non-uniform and hence the peripheral portion of the semiconductor wafer is excessively polished to cause edge rounding. In order to prevent such edge rounding, there has been used a substrate holding apparatus in which a semiconductor wafer is held at its peripheral portion by a guide ring or a retainer ring, and an annular portion of a polishing surface that corresponds to the peripheral portion of the semiconductor wafer is pressed by the guide ring or the retainer ring.
A thickness of a thin film formed on a surface of a semiconductor wafer varies from position to position in a radial direction of the semiconductor wafer depending on a film deposition method or characteristics of a film deposition apparatus. Specifically, the thin film has a film thickness distribution in the radial direction of the semiconductor wafer. When a conventional substrate holding apparatus for uniformly pressing an entire surface of the semiconductor wafer is used for polishing the semiconductor wafer, the entire surface of the semiconductor wafer is polished uniformly. Therefore, a conventional substrate holding apparatus cannot realize a polishing amount distribution that is equal to the film thickness distribution on the surface of the semiconductor wafer, and hence cannot sufficiently cope with the film thickness distribution in the radial direction so as to cause insufficient or excessive polishing.
As described above, the film thickness distribution on the surface of the semiconductor wafer varies depending on the type of a film deposition method or a film deposition apparatus employed. Specifically, a position and number of portions having a large film thickness in the radial direction and difference in thickness between thin film portions and thick film portions vary depending on the type of a film deposition method or a film deposition apparatus employed. Therefore, a substrate holding apparatus capable of easily coping with various film thickness distributions at low cost has been required rather than a substrate holding apparatus capable of coping with only a specific film thickness distribution.